Elastin-like polypeptides (ELPs) are stimulus-responsive protein-based biopolymers, and some ELP block copolymers can assemble into spherical nanoparticles with thermosensitivity. In this study, two different ELP diblock copolymers, each composed of a hydrophobic and a charged moiety, were synthesized, and the dependence of their physical properties on pH, temperature, and salt concentration was investigated. A series of analyses revealed that hydrophobic core micelles could be generated in response to a change in their surroundings and that micelles did not self-aggregate, a phenomenon due to the repulsive forces between like-charged molecules on the surface. We also demonstrated that self-assembly behavior was closely dependent on the character of the charged amino acid and the specific anion in solution.